1. Field of the Invention
This invention relates to methods and apparatus for producing foamed thermoplastic, e.g., polystyrene or polyethylene, articles of relatively large thickness and of relatively large lateral cross-section, e.g., 24 square inches or more, and preferably having a density of not more than 4 pounds per cubic foot.
2. Description of the Prior Art
This invention solves a problem that has plagued the production of large-sized structural plastic foam members for almost three decades. "Large-size structural plastic foam members" include plastic foamed boards, planks, panels, beams and the like of substantial dimensions measuring several inches thick and wide and a few to several feet long used primarily for building insulation, cushioning, flotation and sound deadening.
One of the earliest patents concerning large size structural plastic foams in U.S. Pat. No. 2,515,250 which describes the method comprising extruding a foamable gel into a large pressure-resistant vessel (pressurized at 350 psi with normally gaseous blowing agent and air) for 20 to 24 hours to fill it and then emptying the vessel through a discharge valve into a zone of lower pressure, e.g., atmospheric pressure to form a foamed log or beam. This method was soon criticized in U.S. Pat. No. 2,669,751 (by a different inventor of the same assignee) as being "less flexible in operation than is desired for many purposes" and because "during extrusion of the resultant gel the latter tends to cling to inner walls of the vessel and funnel downward so that vapors escape through the bottom discharge valve when a large amount, e.g., one third or more of the gel remains in the vessel." U.S. Pat. No. 2,669,751 further discloses that the use of a storage vessel, or other device, e.g., a heat exchanger, to condition or age a mixture, or body, of a normally gaseous agent and a thermoplastic polymer so as to obtain a homogeneous mobile gel having a uniform temperature throughout its mass, prior to extrusion of the same to form a cellular product, does not allow as great a flexibility in the operation of the process as is desired, particularly with regard to rapidly effecting changes in the composition, or the temperature, of the mixture being expanded to form the cellular product and notes that the operation takes three days to a week to carry out.
As a correction of these problems, U.S. Pat. No. 2,669,751 proposes the production of foamed polystyrene billets or logs having a diameter of 2 to 2.5 inches by extruding a heatplastified mixture of polystyrene and blowing agent and passing it through a mixing and cooling device to lower the temperature into the range of 140.degree. F. to 266.degree. F. before extruding it from the device into an area of lower pressure where foaming takes place. While some of the problems of U.S. Pat. No. 2,515,250 may be corrected by U.S. Pat. No. 2,669,751, new ones arise in that the extrusion of larger billets or logs is difficult in the equipment disclosed.
As pointed out in U.S. Pat. No. 3,817,669, with a given thermoplastic resin such as polystyrene, one can readily extrude a small round foamed rod, such a rod having a diameter of one to two inches of very low density foam. If one attempts to employ the same or similar conditions and proportions of blowing agent, feed, temperatures and the like to extrude a foam body of larger cross-section; for example, a billet twelve inches in thickness and twenty-four inches in width, one can be eminently unsuccessful; the product can warp, twist and distort.
Larger die orifices are needed to make the larger cross-sectional members. The larger die orifices must be supplied with substantially larger quantities of the foamable mixture within a given time period to avoid foaming in the orifice and the resultant bad surface effects. Standard extruders commercially available are not able to supply enough foamable mixture at the appropriate temperature to satisfy the needs of the large die orifice for avoiding foaming in the orifice. While specially designed, giant extruders conceivably can provide an adequate supply of foamable mixture to the die orifice, the capital expenditures, energy requirements and start-up and running complications can seldom, if ever, be justified by local demand. Transportation costs because of increased energy costs render long distance shipment unattractive or impractical.
No prior art is known which teaches the production of large size plastic foam structural members except by the use of a large pressure-resistant vessel as taught by U.S. Pat. Nos. 2,515,250 or 2,774,991 with its attendant problems or the use of excessively large size equipment which can be difficult to operate and uneconomical and which requires considerable local demand for justification.
Injection molding of polystyrene foam is described by Zielinski in an article in the January 1962 issue of Plastics World ("Injection Molding Expandable Polystyrene Beads", pages 18-20). In the Zielinski process, foamable polystyrene beads, which contained pentane as a blowing agent, were injection molded to produce injection molded parts. Densities of the resulting injection molded foamed polystyrene are substantially always at least half the density of the solid polystyrene employed, e.g., 30 or 40 pounds per cubic foot.
Since the Zielinski article, a great deal of activity has occurred in the injection molding of foamed thermoplastic articles. However, because of the need for the molten polymer-blowing agent mixture to flow into the intracacies of the mold the polymer mixture is kept at a high temperature, e.g., 300.degree. F. to 350.degree. F. or higher during extrusion and relatively high density foams result. If, pursuant to this invention, the polymer-blowing agent mixture were held in the accumulator at a temperature of this magnitude and ejected into a zone of atmospheric pressure, foaming would take place inside the die resulting in foamed articles having a very irregular, rough and unsightly surface (moonscape appearance) and structure.
Angell, Jr., U.S. Pat. No. 3,268,636 discloses a form of injection molding wherein a foamable polyethylene or foamable polystyrene is extruded into an accumulator and, when a sufficient quantity has been collected in the accumulator, the foamable thermoplastic therein is injected into a closed mold. This particular Angell patent refers to the production of a relatively thick article but in the Angell system the foamable composition explodes into small particles which fill the mold and fuse together in the mold to form the article. The densities of the resulting Angell foamed article are quite high and the Angell procedure is not capable of producing foamed articles having densities in the vicinity of four pounds per cubic foot or less. Furthermore, the fused particles which constitute the interior of the article are lacking in strength.
In an improvement patent, U.S. Pat. No. 3,436,446, Angell discloses the injection molding of relatively thin walled articles, such as, receptacles, by extruding a foamable thermoplastic material into an accumulator. When a sufficient amount has been collected in the accumulator, the foamable composition is injected into a closed mold to form a thin walled foam product having densities of more than half the density of the solid thermoplastic employed to make the foam, i.e., having densities of 30 or more pounds per cubic foot. While the products are foamed, their densities are 15 or more times greater than the densities of the foamed articles made by the present invention.
King patent, U.S. Pat. No. 3,249,660 discloses the injection of molten, foamable thermoplastic into a mold at a pressure at which the composition will not foam and then mechanically expanding the mold to reduce the pressure and allow the foamable composition to foam and expand into the confines of the mold. The densities of the resulting articles, as in the Angell procedure and apparatus, are very high compared to the foam densities obtained by the present invention.
Eyles patent, U.S. Pat. No. 3,162,703 discloses the injection molding of foamed articles which involves the use of polystyrene beads blended with pentane which are passed through a heating block in which the polystyrene remains a warm or softened solid. After leaving the heating block it is passed as a mobile gel into heated accumulator 26. When a sufficient amount has been collected in the accumulator, it is ejected into a closed mold which is subjected to a vacuum. The foamable composition expands within the mold to contact and be shaped by the walls of the mold. The densities of such articles produced by this process are also quite high because the expansion of the foamable composition must be great enough to force the composition into contact with the mold walls.
Cronin et al, U.S. Pat. No. 3,437,722 discloses the transfer molding of foamed articles wherein the foamed thermoplastic such as polystyrene or polyethylene, is extruded into an accumulator from which it is intermittently ejected into a closed mold.
None of the references mentioned hereinabove disclose or suggest the present invention in which a foamable composition is extruded into a holding zone or accumulator maintained at a temperature and pressure which does not allow the composition to foam and periodically ejecting the composition in the holding zone through a die orifice, at a sufficiently high rate to avoid foaming within said holding zone or die orifice but at not so high a rate that the surface of the resulting cellular body is damaged as a result of melt fracture, into a zone of lower pressure, e.g., atmospheric pressure, or less to allow the rapid expansion, unrestrained in at least one dimension, of said composition to form an elongated thermoplastic cellular body preferably having a density of not greater than about four pounds per cubic foot and a lateral cross-section of not less than 24 square inches.